+1 for the Merck Manual, but I go for the MD version plus a dictionary. I thing the home version is dumbed down.
I also have to chime in on the concern over "too many people coming in waving printouts of their Internet 'diagnosis'". It's too easy to get attached to a diagnosis yourself, and convince yourself of symptoms that, viewed objectively by an MD who (presumably) is familiar with a much broader set of possible diagnoses, would differ. As a result you can fool yourseld and your doctor into the wrong diagnosis.
That said, I think that once you and your doctor have settled on a diagnosis, it's your responsibility to learn everything you can about it. Understand what the malady is, and how it works. Read about the proposed treatment and any alternative, including how any side effects and other risks have affected people in the same situation you find yourself.
Once you settle on a treatment, understand the mechanics of any surgery, or chemical action of any medication. That way you'll know better what to expect, and more importantly, how to ensure that you don't accidentally cause other damage to yourself.